1. Field of the Invention
The present invention relates to an impact-type recording apparatus wherein an inked ribbon is forced by a hammer against a piece of paper or other recording medium laid on a platen, to record or print characters thereon.
2. Description of the Prior Art
The daisywheel printer has heretofore been known as an example of this type of recording apparatus. A typical daisywheel printer employs a printer head assembly including: a carriage arranged to travel backward and forward along the printing surface of a piece of paper partially wrapped around a platen; a type wheel or daisywheel having a font formed on the ends of "petals" extending radially from its center hub; a motor for driving the type wheel; a hammer unit; and an inked ribbon; with the wheel, motor, hammer and inked ribbon being disposed on the carriage. The hammer unit is commonly constituted by: a magnetic coil; a hammer capable of impacting characters provided on the periphery of the type wheel by the energization of the coil; and resilient means, such as a spring, for restoring the hammer to its initial position after completion of this energized striking motion. In such a daisywheel printer, characters are printed in the following way. A character to be printed is selected on the basis of data on various printing operations, and the magnetic coil for the hammer is energized so as to drive the hammer and thus cause it to strike the selected character, thereby forcing this character against the paper or other recording medium, via the inked ribbon.
In this case, when the hammer strikes each individual character against the paper via the inked ribbon, a certain level of impact is needed. However, the optimum level of impact depends upon the surface area of each character to be brought into contact with the paper. For this reason, a conventional type of impact printer usually has a built-in ROM (read-only memory) constituting part of a printer control section, and the ROM stores a table of impact control data corresponding to the surface area of each character. The impact of each character is controlled with reference to this table. In general, since the mass of the hammer is constant, this type of impact control is provided by controlling the strike velocity of the hammer, that is, by controlling the electric current flowing in the magnetic coil of the hammer unit.
Once the hammer strikes the character to be printed, it is returned to its initial position by a repelling force inherent in the platen and the resilient force of the resilient means, such as a spring, and collides against a stopper provided for defining the initial position. Since the hammer is returned at a velocity substantially as high as the strike velocity thereof, it collides against the stopper with a heavy impact, and thus a large noise is generated. Therefore, in order to reduce the above-mentioned impact and noise, a conventional type of impact printer commonly employs, as stopper means, a resilient damper made of such a material as rubber having a small restitution coefficient.
However, such a prior-art method suffers from the following drawbacks. Even if such a rubber-made stopper is used with the hammer unit in the aforementioned way, it is difficult to prevent the hammer from striking the stopper with a heavy impact accompanied by a large noise. Specifically, in order that the hammer may be caused to strike a character having a large surface area with a sufficient impact, the hammer must be moved toward the object character at high velocity. This increases the velocity at which the hammer is returned to its initial position, and thus a large impact noise is generated. Similarly, when the velocity of movement of the hammer is to be enhanced for high-speed printing, such an impact noise is further increased.
The above-noted problems are common not only to daisywheel printers but also to all the impact-type recording apparatus mentioned previously in the field of this invention.